This invention relates generally to the field of electrical power distribution, and more particularly, is directed to an apparatus and method for retarding electrochemical type decomposition of the electrical insulation of power distribution cables and thereby prolonging their service life.
Electrical power companies have made wide use of underground distribution, feeder and transmission cables to deliver electrical energy to homes, commercial establishments and industry. One of the problems inherent in underground cable systems is the ingress of moisture into the cable insulating structure, leading to decomposition of the insulation and failure of the cable system. Accordingly, reducing or eliminating the deleterious effects of moisture is of paramount concern to those who design, install or maintain underground power cables and power cable systems.
The typical medium voltage underground power installation operates in the 5,000 to 35,000 volt range (5 to 35 KV) using polyolefin-insulated cables principally of polyethylene (PE), crosslinked polyethylene (XLPE) or ethylene propylene rubber (EPR) material. Cables in these installations are sometimes supplied with an outer protective covering, such as an extruded lead sheath, to prevent the ingress of moisture into the cable insulation. The majority of cables, however, do not have a protective covering and thus the insulation is subjected to the ingress of moisture and its deleterious effects.
In high voltage underground power installations which operate in the 60 to 138 KV range, most cables are supplied with an overall protective covering, such as an extruded polyethylene or polyvinyl chloride jacket, for retarding the ingress of moisture into the cable insulation. Some high voltage cables also use an extruded lead covering.
In underground installations, the cables are buried directly in the ground, installed in ducts or submerged under water. In either method of installation, the cables are exposed to high levels of moisture which leads to rapid decomposition and deterioration of the cable insulation.
It is well known that the intermolecular spaces of polyolefin insulation are relatively large, thus allowing infusion of molecules of moisture or other fluids. This property of polyolefin insulation renders it susceptible to a type of insulation deterioration known as electrochemical tree formation. Electrochemical tree formation is believed to occur in the following manner. Moisture from the environment enters the cable and diffuses throughout the insulation structure via the large intermolecular spaces of the insulation. The high dielectric constant of the moisture tends to assist its movement into areas of the insulation having a lower dielectric constant due to the action of the electric field created by the flow of current through the cable conductor. Furthermore, the moisture tends to break into droplets in the intermolecular spaces and in microvoids and crevices which may be present in the body of the insulation. Electrostatic forces acting on the droplets due to the presence of the electric field, causes the droplets to elongate in the direction of the electric field. As the droplets elongate, the radii of the droplet ends decrease with a corresponding increase in the strength of the electric field at the droplet ends. The strength of the electric field increases to a point where electrical discharge occurs at the ends of the droplets, causing decomposition of the insulation in the region of the discharge. More moisture enters into these regions and the electrical discharge area moves further into the insulation. This process continues throughout the cable insulation, thus creating an electrochemical tree.
Because of electrochemical tree formation in the cable insulation, the breakdown voltage level of the cable is significantly lowered. Often the breakdown level falls below the level of protection provided by surge arrestors in the system and may even fall below the operating voltage level of the system. The latter situation is of particular concern in a high voltage cable system. Thus, electrochemical tree formation in the cable insulation, and the attendant lowering of breakdown voltage, greatly shortens the useful life of the cables. In some cases, the service life is reduced to less then ten years rather than the normal 30 to 40 year service life of cables without insulation deterioration due to electrochemical tree formation.
Because microvoids in the cable insulation are a main place where the formation of electrochemical trees begin, much effort in the prior art has been directed toward the elimination or reduction in size of microvoids during the insulation structure extrusion process. Special extrusion tools and controlled gradient cooling of the insulation structure have succeeded in reducing the number and size of microvoids in the insulation. Furthermore, reducing the number of conducting contaminants in the insulation by special handling and by the use of fine-mesh screens at the insulation extruder has been successful in reducing the number of contaminated areas in the insulation where local high voltage stresses can exist. Reducing the number of contaminated areas helps to retard the formation of electrochemical trees. None of these improvements in the prior art, however, have been successful in sufficiently eliminating microvoids in order to prevent electrochemical tree formation altogether.
Moisture impervious outer coverings, such as extruded lead and aluminum sheaths, provide a means for preventing moisture ingress into the cable insulation as explained above. However, these coverings have proven useful in only a limited number of applications for technical reasons and high cost. The technical reasons have chiefly to do with the high coefficient of thermal expansion of polyolefin insulations. When the temperature of metallic covered cables having polyolefin insulation rises, e.g., due to the current flowing in the cable conductor, the insulation expands and stretches the diameter of the metallic covering. In many cases when this occurs, the elastic limit of the materials in the covering is exceeded. Thus, when the cable cools and the diameter of the insulation returns to its original size, the metal sheath remains expanded. Therefore, a longitudinal channel or void is left between the metal sheath and the cable insulation. If the metal sheath loses its imperviousness to moisture, e.g., due to corrosion, pinhole formation or other mechanical damage, moisture can enter into the longitudinal space under the sheath and subsequently migrate into the cable insulation as explained above.
Where lead is used as the sheath material, an additional disadvantage is present. Lead sheaths tend to develop an insulating corrosive layer on the inside surface of the sheath adjacent to the insulation shield. The presence of a corrosive layer leads to electrical discharges between the lead sheath and the semiconducting layer of insulation shielding material. These discharges can result in premature failure of the cable. A further disadvantage of lead sheaths is that when the diameter of the sheath is increased due to expansion of the polyolefin insulation as a result of heat, the length of the sheath contracts. Contraction of the sheath creates longitudinal forces in the sheath which are high and difficult to control. These forces often result in the sheath being pulled away from cable splice boxes and cable terminations. Thus, the integrity of the cable system is compromised and moisture is permitted to enter the cable at the splice boxes and terminations and can flow along the longitudinal void between the sheath and cable insulation structure.
Cable sheaths made of corrugated longitudinally applied copper or aluminum have recently been used in place of extruded lead or aluminum. Cable sheaths made in this manner are not welded at the longitudinal seam. The metal is folded over itself, leaving a longitudinal opening extending along the entire length of the cable. Even though a sheath made with copper or alumnium has an extruded polyethylene jacket applied over it, moisture can still enter the longitudinal opening and diffuse into the underlying insulation structure. Studies have shown that even if a satisfactory outer covering impervious to moisture is devised, it most likely will be too expensive for general use on power distribution cables.
There are a large number of underground polyolefin insulated cables currently in service which do not have moisture resistant outer coverings. The insulation on many of these cables have deteriorated due to electrochemical tree formation. Accordingly, the service life of these cables will be significantly shorter than normal. Some of these cables, particularly those installed in ducts, can be replaced; but replacement is extremely difficult and costly. Consequently, a more expedient and less costly system of maintaining cable installations is needed. The method and apparatus disclosed herein is considered a more desirable and useful alternative.